1. Field of the Invention
The present invention relates to a resin-made spring which is used as a spring for a check valve disposed in a fluid apparatus such as various kinds of pumps and valves.
Furthermore, the present invention relates also to a bellows type constant volume pump into which a resin-made spring is incorporated, and particularly to a bellows type constant volume pump for transporting a fluid such as chemicals or pure water which is desired to be subjected to constant volume transportation that is free from retention.
2. Description of the Prior Art
A spring which is to be used in a fluid path is required to have the property that it is not corroded or rusted by contact with the fluid, or to have excellent chemical resistance and a rust proof property. As a spring which satisfies such requirements, conventionally, known are a resin-made spring which is produced by forming a fluororesin such as PTFE (Polytetrafluoroethylene) or PFA (Perfluoro-alkoxyfluoro Plastics) into a coil-like shape, and a resin-coated spring which is produced by coating the whole outer surface of a metal-made spring with a fluororesin such as PTFE or PFA.
Particularly, a bellows type constant volume pump which handles a fluid such as chemicals or pure water is strongly required to be excellent in chemical resistance and to have a rust proof property. In a prior art bellows type constant volume pump of this kind, therefore, a resin-made spring which is produced by forming a fluororesin such as PTFE or PFA into a coil-like shape is used as a spring. The spring is incorporated into a check valve that is disposed at a basal end portion of a suction hole of the pump body, or a tip end portion of a discharge hole so as to allow flow only in the suction direction or the discharge direction and inhibit flow in the opposite direction. The spring elastically urges the valve element of the check valve in the closing direction.
However, resin-made spring of the prior art exerts a weak repulsion force (a repulsion strength: a spring force). Even if the repulsion force is of a degree similar to that of a metal-made spring at an initial stage, "setting" due to repetitive use remarkably appears. As a result, the repulsion force is inevitably reduced with time by the creep of the resin, particularly, thermal creep.
According to the configuration of a resin-coated spring of the prior art, it is very difficult to produce a spring of small size because of technical problems, resulting in high production cost. Furthermore, the coating resin may be peeled off, thereby producing a fear that the expected chemical resistance and rust proof property cannot be maintained.
When the above-mentioned coil-like resin-made spring is used as a spring which is to be incorporated into a check valve for a bellows type constant volume pump, there arises the following problem. In the case where the pump is used by placing the discharge port at a level lower than the suction tank, when the discharging pressure is lower than the head pressure of the tank, a siphon phenomenon occurs during a discharge process because of a small spring load value which urges the valve element of the check valve on the side of the tip end portion of the discharge hole in the closing direction. This causes the valve element of the check valve on the side of the basal end portion of the suction hole to unexpectedly open at a pressure which is lower than the head pressure of the tank, thereby suppressing buildup of the internal pressure gradient of the pump. As a result, a larger amount of fluid flows thereby increasing the discharge amount, and the desired constant volume property cannot be ensured. In a coil-like resin-made spring of the prior art, moreover, the repulsion force is reduced with time by creep, particularly, thermal creep. Also for long-term use at either ordinary temperature or a high temperature, therefore, the constant volume property cannot be stably maintained.